1. Field of the Invention
The present invention relates to a switching power supply that substantially reduces GND bounce noise accompanying switching operation of a switching element.
2. Description of the Related Art
A switching power supply is basically composed of a main body SW of the switching power supply and a control circuit (or a control circuit IC) CONT as shown in FIG. 5A and FIG. 6A, for example. The main body SW of the switching power supply performs switching an input voltage Vin by means of a switching element Q to obtain a predetermined output voltage Vout through an isolating transformer T. The control circuit CONT performs ON/OFF driving operation of the switching element Q at a predetermined frequency. FIG. 5A shows an example of circuit construction to detect switching current in a positive detection mode, and FIG. 6A shows an example of circuit construction to detect switching current in a negative detection mode.
The switching element Q is connected in series to the primary winding P1 of the isolation transformer T and performs switching operation of the input voltage Vin. An alternating voltage is generated across the secondary winding S of the isolation transformer T accompanying the switching operation of the switching element Q and rectified through the diode D, and then smoothed with the output capacitor Cout to be delivered as an output voltage Vout. The symbols Cin in FIG. 5A and FIG. 6A designate input capacitors.
The control circuit CONT operates with a driving power source of the voltage generated across an auxiliary winding P2 of the isolation transformer T and controls the switching operation of the switching element Q according to feedback information from an output voltage detecting circuit (not shown in any figures) for detecting the output voltage Vout. The switching control is conducted by means of PWM control in which the ON width of the ON/OFF driving of the switching element Q at a predetermined frequency or a corresponding period is varied according to the feedback information. The control circuit CONT has an overcurrent protection circuit (not shown in any figure) for the switching element Q in which an overcurrent through the switching element Q is detected with a shunt resistor Rs connected in series to the switching element Q.
The switching power supply having a construction described above generates GND bounce noise synchronous with the switching operation of the switching element Q caused by an impedance component due to a circuit pattern between a grounding point E1 of the main body SW of the switching power supply and a grounding point E2 of the control circuit CONT. The impedance component between the grounding point E1 and the grounding point E2 changes, as shown by the equivalent circuits in FIG. 5B and FIG. 6B, depending on the detection mode, the positive detection mode or the negative detection mode, of the switching current through the shunt resistor Rs. The symbol Lpcb designates an inductance component of the circuit pattern or a wiring pattern, and Rs and Ls designate the resistance component and the inductance component of the shunt resistor Rs, respectively.
FIGS. 7A and 7B show GND bounce noise generated in the positive detection mode and the negative detection mode, respectively. It is apparent that the magnitude of the GND bounce noise is remarkably different between the two detection modes caused by the difference of impedance between the grounding point E1 and the grounding point E2. Paying attention to the noise level synchronous with the switching frequency, which is around 60 kHz, of the switching element Q in particular, the peak level of the GND bounce waveform in the negative detection mode is about 3.5 times of the one in the positive detection mode. In addition, a spectrum of the GND bounce noise, though not shown here, exhibits a difference in peak values as large as 10 dB.
When the switching power supply is subjected to an external noise, for example, an aerial discharge of −15 kV, the output voltage Vout of the switching power supply changes as shown in FIGS. 8A and 8B. In the case of the negative detection mode, the output voltage Vout shows a large bounce from a rated output voltage of 19 V to about 5V. When the external noise that causes such variation in the output voltage Vout is superimposed on the GND bounce noise accompanying the switching operation, a malfunction may occur in the control circuit CONT.
A technique for eliminating the noise generated on the ground line is disclosed, for example, in Patent Document 1 (identified below) in which a noise canceling circuit is provided on an output stage of an LSI and canceling current is delivered to the ground line of the LSI at the moment of transition from an H level to an L level of the output stage. In a technique disclosed in Patent Document 2 (identified below), a common mode current is detected from the input current of an inverter device for driving a three-phase motor and the canceling current that is generated corresponding to the common mode current is fed to the ground line of the inverter device.
[Patent Document 1]
Japanese Unexamined Patent Application Publication No. H10-126237
[Patent Document 2]
Japanese Unexamined Patent Application Publication No. 2006-333647
In the noise canceling technique disclosed in Patent Document 1, the noise canceling circuit generates a canceling current in the reversed phase with respect to the current flowing in the output stage based on the control signal for driving the switching element in the output stage. As a consequence, the noise canceling circuit would necessarily include a switching element having the same characteristic as that of the switching element in the output stage. Thus, the GND bounce noise can hardly be canceled in the switching power supply.
In the noise canceling technique disclosed in Patent Document 2, a canceling current is generated by detecting a common mode current from the input current of the inverter device and the canceling current is obtained from a grounded current in the three-phase motor of the load side.
As a consequence, the inverter device needs to be connected to the outer case of the three-phase motor in the load side. Thus, the overall construction becomes complicated. Moreover, the input terminal of the inverter device needs to be interposed with an inductor and a current transformer for detecting the common mode current, which further complicates the circuit construction.